What is opioid-induced hyperalgesia?

We all have normal levels of heightened sensitivity to various stimuli. For example, you may have a favorite smell. Perhaps, it is cigar smoke because it fondly reminds you of your grandfather. A slight whiff of cigar smoke makes you think of your grandfather and gives you pleasant feelings. This experience is in response to a simple smell, which to others, leads to no such connotations. Or, perhaps, you have a favorite song. You may find yourself on an elevator and upon hearing the elevator music, you recognize the song and it feels good. No one else in the elevator even notices the song until you say, "Oh, that’s my favorite song." To take yet another example, you might be able to readily pick out your spouse in a large crowd and subsequently come to feel a sense of warmth and joy at simply your spouse’s presence. To others, however, your spouse may simply be an invisible stranger among others in the crowd. These kinds of normal sensitivity can be present in any and all of the senses, including touch, sight, smell, taste, and hearing.